Here's some info from MedHelp - sone linked posts look interesting
Small black flecks/organisms/stringy things in stool ...
I always have small black stringy things in my stool; they are usually less than 1 ... Testing the stool for infection (i.e. ova and parasites as well as bacteria), bleeding and ...
Most worm ova are too small to be seen tho maybe you could see Fasciiola gigantium. I looked at Isospora but it's not dark tho something in the stool coulld have stained it but I may be too small to see. Since yeasts look like tiny white balls in clusters and strings and thought it might be spores of a black mold.